1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device and a method for manufacturing the same.
2. Description of the Related Art
A related art nitride semiconductor light-emitting device includes a nitride semiconductor material having the empirical formula represented as AlxInyGa(1-x-y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1). In particular, researches for a semiconductor light-emitting device using gallium nitride (GaN) are being actively advanced currently. For example, a sapphire substrate, which is an insulating substrate, is generally used up to now because there does not yet exist a commercial substrate having the crystal structure identical to and lattice-matched with the nitride semiconductor material such as GaN in the nitride semiconductor light-emitting device.
Between the sapphire substrate and the GaN layer grown thereon, a lattice mismatch occurs due to differences between respective lattice constants and between respective thermal expansion coefficients. Thus, to prevent this lattice mismatch, a GaN buffer layer, which is grown at a low temperature, is formed on the sapphire substrate, and thereafter a GaN layer is grown on the buffer layer at a high temperature. The reason is to reduce the lattice constant difference between the sapphire substrate and the GaN layer.
However, the GaN buffer layer grown at the low temperature has many crystalline defects, and it has amorphous property rather than crystalline property. Therefore, if the GaN layer is directly grown on the low temperature growth buffer layer at the high temperature, many crystalline defects such as dislocations propagate into the high temperature growth GaN layer. In the related art, to grow a dislocation-free GaN layer, there is employed a lateral epitaxial overgrowth (LEO) method or a pendeo-epitaxy method.
FIGS. 1A to 1D are schematic sectional views illustrating a method of growing a GaN layer using the LEO method according to the related art.
Referring to FIG. 1A, a GaN epitaxial layer 11 is grown on a sapphire substrate 10 first. Hereinafter, the GaN epitaxial layer 11 grown on the sapphire substrate is referred to as a primary GaN epitaxial growth layer 11. Thereafter, referring to FIG. 1B, a mask 12 having a predetermined pattern is formed on the primary GaN epitaxial growth layer 11, wherein the mask 12 is formed of silicon oxide, silicon nitride and so on.
Afterwards, referring to FIG. 1C, a GaN layer 13 is regrown on a portion where the mask 12 is not formed. More specifically, the GaN layer 13 is grown laterally as indicated as an arrow of FIG. 1C. When the lateral growth of the GaN layer 13 is completed, the growth of the GaN layer 13 is completed as illustrated in FIG. 1D.
Meanwhile, a pendeo-epitaxy method is similar to the LEO method. That is, the pendeo-epitaxy method includes: epitaxially growing a primary GaN epitaxial layer on a sapphire substrate; forming a mask on the primary GaN epitaxial layer; removing the primary GaN epitaxial layer over which the mask is not formed using an etching process; and regrowing a GaN epitaxial layer over a groove after growing the groove.
In general, it is known that the number of dislocations propagating in the GaN layer formed by the LEO method or the pendeo-epitaxy method is reduced.
Referring to FIG. 2, the dislocation A existing under an exposed portion of the primary GaN epitaxial growth layer 11 propagates into the regrown GaN layer 13. However, the dislocation existing under a portion of the primary GaN epitaxial growth layer 11 covered with the mask 12 does not propagate into the regrown GaN layer 13 because the GaN layer 13 over the mask is grown laterally. Thus, the defect can be somewhat decreased in virtue of the lateral growth.
However, when growing the GaN layer according to the related art, besides the dislocation A in the primary GaN epitaxial layer uncovered with the mask 12 propagates upward, there exists a problem that high density dislocation B occurs at a contact surface where the GaN layers 13 laterally grown from both sides of the mask 12 meet together.
In addition, there is another problem that defects occur due to the stress formed between the mask 12 and the regrown GaN layer 13. These defects such as dislocations or the like cause the electrical and optical properties of the nitride semiconductor device to be degraded, which lead to a yield drop after all.
Furthermore, the manufacturing cost increases inevitably because the related art LEO or pendeo-epitaxy method requires a process of preparing a mask. Moreover, the manufacturing process is too complicated because the patterning process and regrowing process should be additionally performed after growing the primary epitaxial layer.
Even though the LEO method or the pendeo-epitaxy method is used for reducing the defects caused by lattice mismatch, it is difficult to remarkably reduce the defects such as dislocations according to the related art. Instead, the process becomes complicated and the manufacturing cost is increased due to the additional processes.
Therefore, there are required a new nitride semiconductor light-emitting device with enhanced electrical and optical properties and a manufacturing method thereof in this technical field, which can prevent the defects such as dislocations caused by the lattice mismatch between the sapphire substrate and the nitride semiconductor material, e.g., GaN.